This application claims the priority of German Application No. 198 25 829.1, filed Jun. 10, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for determining the position of a structural element on a substrate, wherein the substrate is mounted on a measuring stage. The measuring stage is displaceable in an interferometrically measurable fashion in a measuring plane relative to a reference point. The structural element is imaged on a detector array by an imaging system that has its optical axis perpendicular to the measuring plane. The pixels of the detector array are arranged in rows and columns parallel to the axes of an X/Y coordinate system associated with the substrate. The position of the structural element is defined by the distance of one edge of the structural element relative to the reference point, and the position of the edge is determined on the detector array by evaluating an intensity profile of the edge image that is perpendicular to the edge direction and is derived from pixels located in a defined measuring window of the detector array.
A metrology system suitable for performing the method, such as Leica""s commercially available LMS IPRO(copyright) measurement system is described, for example, in the text of the paper xe2x80x9cPattern Placement Metrology for Mask Making,xe2x80x9d by Dr. Carola Blxc3xa4sing, delivered in the Education Program of Semicon Genf on Mar. 31, 1998, the disclosure of which is expressly incorporated by reference herein.
The structural elements (structures) to be measured consist in particular of opaque or transparent areas (such as patterns) on mask surfaces or structures on wafers or reticles used in semiconductor manufacturing. The positions of the edges of the structural elements are measured in a coordinate system defined on the mask (mask coordinate system). The mask is mounted in the measuring machine on a measuring stage (such as an X/Y stage) that is displaceable in a measuring plane. The measuring stage is displaceable relative to a reference point in an interferometrically measurable fashion, with the position of the mask coordinate system being aligned using alignment marks relative to a machine coordinate system of the measuring machine. Usually, the contact point of the optical axis of the imaging system on the mask is used as the reference point.
A structural range to be measured, following a suitable displacement of the measuring stage, is imaged (enlarged by the imaging system) on a detector array of a CCD camera. The pixels of the CCD camera are arranged in rows and columns parallel to the aligned axes of the mask coordinate system. Conventionally, the edges of the structural elements to be measured are likewise parallel or perpendicular to the axes of the mask coordinate system and hence also to the rows and columns of the detector array. The edge position that results from the evaluation of the image of the structural element taken by the detector array is provided by the interpolated pixel rows or pixel columns on which the edge lies relative to the reference point. The detector array is generally aligned such that the center of its camera screen image lies on the optical axis of the imaging system, so that this center (screen origin) is used as the reference point.
The image of the structural element is evaluated using image analysis process methods. A specific array range is selected for the measurement with the aid of a rectangular measurement window (sometimes referred to as a measurement field) defined and generated by software. The measurement window is placed on a portion of the image of the structural element to be measured. As a result of the resolution and imaging quality of the imaging system, the degree of contrast of the edge image varies. The best contrast is set with the aid of a TV autofocusing system. An average value is formed from the intensities of the pixels that lie in a row or column parallel to the edge of the structural element within a measuring window. Perpendicular to the edge, this produces an intensity profile of the edge image over one pixel row or pixel column. The position of the edge is defined by the 50% level value of this intensity profile.
The structural elements to be measured have different widths and lengths. In order to specify the position of a structural element on the mask (such as the pattern placement), the edge lengths that are parallel to one another are frequently measured and the center line (or midpoint) between the two edges is given as the position. In a structural element that can be measured by the width and length within the measuring window formed by the detector array or in two intersecting structural elements detected using two measuring windows, the position of the structure is defined by the coordinates of the intersection of the two center lines through the windows.
To an increasing degree, structural elements used in designing semiconductor circuits no longer extend parallel or perpendicular to the mask coordinate system. The image that results from the detector array representing these non-orthogonal structural elements is therefore likewise no longer parallel to or perpendicular to the pixel rows and pixel columns of the detector array.
There is therefore needed a system and method for measuring structural elements having angles other than parallel or perpendicular to the coordinate system (so-called xe2x80x9cnon-orthogonalxe2x80x9d elements).
By rotating the CCD camera or the measuring stage, these structural elements can again be aligned orthogonally to the pixel rows and columns of the detector array. The rotational angle can be measured and hence the position of the edge or the position of the structural element can be calculated back to the non-rotated coordinate system so that the measuring machine for orthogonal and non-orthogonal structural elements can perform measurements directly using the same evaluation method and can output the measurement results in a comparable form.
One disadvantage of this method is the high mechanical cost required for precise mounting when rotating the CCD camera or the measuring stage. In addition, the rotation itself and the alignment with the edge requires additional time expenditures which lengthens the time required for measurements. With the ever increasing structural density and number of structural elements to be checked by measurement, however, limiting the amount of processing time has steadily grown in importance.
Hence, the goal of the invention is to provide a method and measuring machine that can be used on structural elements aligned in any direction while not requiring mechanical changes in the measurement process within a measuring field.
This goal is achieved according to the present invention by providing a method in which a rectangular measuring window is produced and aligned with a boundary line parallel to the edge direction, the direction of the boundary line of the measuring window that is perpendicular to the edge direction is determined in the coordinate system of the detector array by its rotational angle xcex8, and in the case of the boundary lines of the measuring window that are not orthogonal to the rows or columns of the detector array, a virtual array is formed whose fields lie in rows and columns parallel to the boundary lines of the measuring window. Intensity values are assigned to the fields that are determined by a weighted evaluation of the intensities of the pixels in the detector array, each of the pixels being covered by a field. The position PIPC of the edge is determined from its distance to the reference point, using the intensity values associated with the fields, and the position P of the edge in the coordinate system of the substrate is given as a function of the rotational angle xcex8, the position PIPC, and the x,y position of the reference point in the coordinate system of the substrate by the equation: P=PIPC+L, wherein L=xxc2x7cos xcex8+yxc2x7sin xcex8.
For automatic alignment of the boundary line of the measuring window parallel to the edge direction, the measuring window can be internally divided perpendicularly to the edge direction. Measuring information is determined in each measuring window that defines the edge position. The measuring window is turned until an optimum correlation of the measuring information is achieved. The size of the virtual fields is advantageously adjusted to that of the pixels. The adaptation is advantageously performed as a function of the rotational angle xcex8.
The method according to the invention is based on the same measuring information which the known measuring machine produces. The basic measuring principle of deriving the edge position within a rectangular measuring window from the pixel intensities is retained. The method therefore has the advantage of being able to be retrofitted to existing machines, such as the current LMS IPRO(copyright) System.
The rotational angle of the measuring window, generated by software, in the predetermined coordinate system of the measuring window can be calculated with a high degree of accuracy and is used as an additional parameter in producing measured values. This parameter is also implicitly contained in the known method for orthogonal structural elements and in that case has a fixed value of 90xc2x0 with the x coordinate and a value of 0xc2x0 with the y coordinate. This facilitates linking the method for determining positions of non-orthogonal structural elements according to the invention with the method used previously.
A critical step is the transfer of the measured intensity values of the real (actual) pixels which are overlapped by the selected, rotated, measuring window into a virtual pixel array that is orthogonal to the boundaries of the rotated measuring window. The intensity values associated with the virtual pixels (fields) are obtained by performing a weighted evaluation of the real pixels which are overlapped. Within the coordinate system of the measuring window, the position of the edge of the structural element can again be determined by the known image analysis method. In a mathematical interpretation, the edge found is a straight line in the rotated coordinate system of the measurement window, so that the distance of the edge from a reference point in the measuring window of the real detector array can be determined from the rotational angle of the measuring window by using coordinate transformation. The optical axis of the measuring system is selected as the reference point which passes through the center of the measuring field formed by the detector array.
The position of the structural element in the mask coordinate system is obtained, with the distance of the reference point from the origin of the mask coordinate system being determined at the same time. This distance clearly depends on the current measuring stage coordinates and the rotational angle of the measuring window. The position of the edge is thus defined by the distance of a straight line from the origin of the mask coordinate system to the edge and the angle at which this straight line intersects an axis of the mask coordinate system.
The positions of two structural elements that intersect at any angle can therefore be defined and determined by the intersection of the straight lines associated with them. The same also applies to the position of a structural element that is aligned in any direction and whose width and length can be measured.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.